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The next item I installed was a boost gauge. I knew that I would eventually
start playing around with boost levels, so I needed to know what level of boost
I was currently running and to know how much further I could boost it without
excessive risk. The installation of the gauge was relatively straight-forward,
with the general advice from the Web Forums being to plumb the gauge into the
charcoal canister purge line. Once the installation was complete and I took
the car for a test run, I found it curious that while the gauge would display
boost fine, the vacuum reading did not behave as I expected. It would give a
vacuum reading only on part throttle. A fully closed throttle would result in
the gauge returning to zero. Close inspection of the throttle body showed that
the port used to purge the charcoal canister is actually located on the outside
of the throttle, which meant that when closed, the stock recirculation valve
(otherwise known as a blow-off valve) would open and release all pressure in
the intake tract before the throttle, returning the air pressure to ambient
(zero on the gauge). Where I should have plumbed the gauge was into a vacuum
line that is located on the plenum side of the throttle. I then swapped the
connection over to the vacuum line that is also connected to the fuel regulator.
This unfortunately also meant I had to replace the charcoal canister hose as
I had tapped into the line some distance from the end, which did not leave me
enough length to reconnect it without a joiner (which would cost more than a
replacement hose).
It was interesting to note that the boost readings I was getting were
considerably higher than what is quoted in the texts for standard boost (6-7
PSI). The maximum boost I was seeing was around 9-10 PSI, with boost beginning
at around 2000 RPM and full boost available at around 3000 RPM. These figures
are only approximate as the car is accelerating very hard at this point and
you really can't take your eyes off the road to read this accurately! As I
knew there was no boost controller installed in the car, the only explanation
for the levels is the two modifications that the car did have - the exhaust
and pod filter. Discussions on the web forums seems to corroborate this.
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